the sending unit for the oil pressure, is located down on the passenger side of the motor, next to the oil filter. sometimes you can just take it out and clean it with some carb cleaner, make sure you use thread tape to put it back in. other times its just the connection of the wire on top that needs cleaned and adjusted.
Or it needs replaced.
But dont be surprised if you put a new one in and get the same readings. The guages in a YJ are noted for being good for awhile then they do their own thing, sometimes they work others they go on vacation or something...lol
Good luck guys.